How do ı calculate the harmonics by inspecting the signal in matlab?

such as this signal gf=cos(2*pi*10*t)+ 0.5*sin(2*pi*20*t)+4*cos(2*pi*30*t-pi/3)

Answer 1

[Rick Rosson]

   dt = t(2) - t(1);
   Fs = 1/dt;

   pwelch(gf,[],[],[],Fs);